In these days, research and development of optical functional devices have been intensively carried out to process a large amount of information in a high speed. Among the optical functional devices, much attention has been paid to an optical bistable device to be utilized as a key element in an optical information processing system. For this purpose, various kinds of optical devices have been proposed. In principle, all kinds of optical input and output logic devices can be provided by combining an inverter and an AND circuit. In this point, less number of proposals have been made on an inverter for inverting an input light, from which an output light is not emitted when an input light is supplied thereto, and an output light is emitted when no input light is supplied thereto, although various types of AND circuits have been proposed. One of such inverters is described on pages 393 to 400 of "the report No. J-66(5), 1983, in the Institute of Electronics Informations and Communications Engineers". The inverter comprises a photodiode for receiving an input light, a first light emitting diode connected in series to the photodiode to provide the photodiode with a positive feedback light, a second light emitting diode for emitting an inverted output light, a first resistance connected between a power supply and the second light emitting diode to provide a predetermined saturation characteristic for the output light, a second resistance connected between the photodiode and the ground for setting a threshold value between the two stable states, and a diode connected in series to the second light emitting diode to minimize a light output in the OFF state. The structure of this inverter will be explained in more detail later.
In operation, where no input light is supplied to the photodiode, a reverse bias voltage is applied to the photodiode, so that almost all of a power supply voltage is applied to the photodiode. As a result, no feedback light is supplied from the first light emitting diode to the photodiode, because no current flows substantially through the first light emitting diode. On the other hand, a forward bias voltage is applied across the second light emitting diode and the diode in series connected, so that a current flows through the second light emitting diode, from which an inverted light output is thereby emitted.
On the contrary, where an input light is supplied to the photodiode, a current flows through the photodiode, so that a feedback light is supplied from the first light emitting diode to the photodiode by a portion of the current flowing through the photodiode. Thus, the current is increased to provide an increased voltage drop across the first resistance so that a voltage is decreased to be applied across the second light emitting diode and the diode in series connected. As a result, an output light which is emitted from the second light emitting diode is decreased. In this state, it is possible that no output light is emitted from the second light emitting diode by setting a built-in voltage of the diode, a voltage of the power supply, and a value of the first resistance to be appropriate levels. Consequently, an optical inverter is realized to provide the two stable states.
However, the above described optical inverter has disadvantages in that the adjustment of a functioning point, etc. is complicated because the element number and kinds such as the photodiode, the light emitting diodes, the diode, and the resistance, etc. are difficult to be decreased, and the isolation of light is absolutely necessary to be carried out, because the feedback of light is utilized to provide an optical bistable device. A further disadvantage is observed in that a light input is necessary to be supplied to the photodiode in providing a light output from the inverter, because the light output is turned off, where the light input is turned off. As a result, an electric power consumption becomes large for a light source, where inverters are arranged in a matrix pattern on a plane of an integrated device.